White phosphorus (P-4) reacts with three-coordinate molybdenum(III) trisamides or molybdaziridine hydride complexes to produce either bridging or terminal phosphide (P3-) species, depending upon the ancillary ligand steric demands. Thermochemical measurements have been made that place the Mo equivalent to P triple bond dissociation enthalpy at 92.2 kcal center dot mol(-1). Thermochemical measurements together with computational analysis rule out simple P-atom abstraction from P-4 as a step in the phosphorus activation mechanism. Kinetic measurements made by the stopped-flow method show that the reaction between the monomeric molybdenum complexes and P-4 is first-order both in metal complex and in P-4. cyclo-P-3 complexes can be obtained when ancillary ligand steric demands are small, but kinetic measurements rule them out as monometallic intermediates in the P-4 activation mechanism. Also studied by calorimetric, kinetic, and in one case variable-temperature NMR methods is the process of p-phosphide bridge formation. Post-rate-determining steps of the P-4 activation process were examined in a search for minima on the reaction's potential energy surface, leading to the proposal of two plausible, parallel, bimetallic reaction channels.